Method for Operating a Reducing Agent Supply System

ABSTRACT

A method for operating a reducing agent supply system for supplying an exhaust aftertreatment system of a motor vehicle is provided. In normal metering operation of the reducing agent supply system an at least approximately continuous air stream and a pulsed reducing agent stream are produced and are released at least partially via the nozzle into the exhaust line. In a cleaning operation for cleaning the reducing agent supply system an air stream formed from a number of air stream pulses and a reducing agent stream formed from a number of reducing agent stream pulses are produced and supplied to the nozzle means. The method can be used in particular in supply systems in which aqueous urea solution is used as reducing agent.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for operating a reducing agent supply system a reducing agent supply system for supplying an exhaust aftertreatment system of a motor vehicle with a reducing agent serving for emission control, in which an air stream and a reducing agent stream are supplied to a nozzle which opens into an exhaust line.

In order to reduce nitrogen oxides in the exhaust of internal combustion engines, in particular of motor vehicle diesel engines, it is known to add aqueous urea solution to the exhaust. In the hot exhaust, ammonia is released which, as a selectively operating reducing agent, renders nitrogen oxides harmless by reduction at a selective catalytic reduction (SCR) catalyst. The urea solution is frequently, by means of compressed air support, supplied to the exhaust in atomized form as an aerosol. One problem of such methods is that malfunctions can be caused due to deposits of urea in the corresponding reducing agent supply system. In order to overcome these and to clean the reducing agent supply system, DE 10 2006 007 658 A1 discloses that, in addition to the air and to urea solution used, a third medium, preferably a solvent for urea, is used for flushing out deposits of urea. The provision of the third medium, however, involves an unwanted expense.

Exemplary embodiments of the present invention provide a method for operating a reducing agent supply system that permits simplified cleaning of the reducing agent supply system.

The method according to the invention is distinguished in that in normal metering operation of the reducing agent supply system, an at least approximately continuous air stream and a pulsed reducing agent stream are produced and released at least partially via the nozzle into the exhaust line, and in a cleaning operation for cleaning the reducing agent supply system, an air stream formed from a number of air stream pulses and a reducing agent stream formed from a number of reducing agent stream pulses are produced and are supplied to the nozzle. An air stream pulse or reducing agent stream pulse in this case is to be understood to mean a sudden and considerable change in the air flow rate or in the reducing agent flow rate. In particular, an air stream pulse or reducing agent stream pulse consists of briefly switching on an air stream or a reducing agent stream such that the air stream or the reducing agent stream suddenly changes from a switched-off state to a switched-on state and then back into the switched-off state. The duration of a pulse lies preferably in the range from one to about twenty seconds.

Although the method for the operation of supply systems is suitable for different, in particular liquid, reducing agents, such as for example mineral oil, alcohol, hydrocarbons and the like, its use in supply systems for supplying an exhaust aftertreatment system with aqueous urea solution (HWL) is particularly advantageous and preferred.

When using HWL as reducing agent, in normal metering operation pulses of HWL that are spaced apart from each other timewise are preferably supplied to the approximately continuous air stream such that an aerosol-like mixture of fine HWL liquid droplets and air is formed. The HWL stream can be supplied to the air at the location of the nozzle, for example close to the exit opening. In particular in such case, part of the HWL stream can be returned from the nozzle back to a storage vessel for HWL, so that only part of the HWL stream is released into the exhaust. Preferably, however, a pulsed HWL stream is supplied into the at least approximately continuous air stream at a point remote from the nozzle, and the mixture is supplied via a mixture line to the nozzle which opens into the exhaust line, and is added to the exhaust stream. The HWL pulses in this case are preferably calculated according to requirements such that overall a desired NOx conversion in an NOx reduction catalyst of the exhaust aftertreatment system is at least approximately achieved. The production and admetering of the pulsed HWL stream preferably takes place by pulse width modulated operation of a metering valve. The air stream is preferably delivered by a compressed air supply unit.

If a need for cleaning of the reducing agent supply system is detected, then, preferably after positive checking of pre-set enabling conditions, a cleaning operation is activated. In the cleaning operation, both air stream pulses and HWL stream pulses are produced and supplied to the nozzle. As a consequence of the spasmodic action of air, HWL and/or HWL/air mixture resulting therefrom on deposits possibly present in the reducing agent supply system, the effective removal and expulsion thereof from the reducing agent supply system is made possible. Of course, provision may also be made to activate the cleaning operation as a preventive measure at pre-settable times, for example upon each second, each fifth, or generally each nth start-up of the reducing agent supply system.

In one embodiment of the method, a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle is determined and the cleaning operation is activated upon exceeding a pre-settable, in particular first, upper pressure threshold value. Reliable monitoring of the reducing agent supply system, in particular with regard to deposits and other contaminants having the effect of constricting the cross-section in the line system, is made possible due to preferably continuous pressure monitoring. For example, deposits due to corrosion or alternatively urea deposits in the line system through which air flows of the reducing agent supply system result in an increased resistance to flow. In the case of an air stream of known or pre-settable size which is preferably used, this results in a measurable pressure increase. By monitoring the back pressure at a pre-set suitable point in the line system, preferably in a line through which air and/or HWL/air mixture can flow, it can thus be reliably determined whether there is a need for cleaning of the reducing agent supply system. If an impermissible increase in the back pressure is detected, the cleaning operation is activated.

Analogously, in a further embodiment of the method, a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle is determined and an activated cleaning operation is deactivated if the pressure drops below a pre-settable lower pressure threshold value. As a result of the pressure monitoring while the cleaning operation is running which is provided according to the invention, it can reliably be determined whether and at what time the cleaning operation has been successful to the desired extent. This is typically manifested in a reduced resistance to flow which is brought about by removal of deposits, which resistance is recognized by a pressure measurement. The cleaning operation is thereupon deactivated, and optionally there is a return to normal metering operation, or the latter is activated. The normal metering operation is thus interrupted for only as long as is absolutely necessary, and unnecessarily long maintaining of the cleaning operation is avoided.

In a further advantageous embodiment of the method, an activated cleaning operation is deactivated after exceeding a pre-settable total value of amount of reducing agent of the reducing agent stream formed from reducing agent stream pulses or after exceeding a pre-settable duration from the beginning of the cleaning operation. This is advantageous in particular, but not only, in the case of activation of the cleaning operation as a preventive measure, since total amounts of reducing agent or durations of the cleaning operation which are effective for cleaning can be determined beforehand and thus can be optimally set.

In a further embodiment of the method, a first operating mode is provided for the cleaning operation in which a sequence of reducing agent stream pulses and air stream pulses is produced such that a particular reducing agent stream pulse overlaps timewise with a particular air stream pulse. This makes it possible for only air or only liquid reducing agent at certain times, and a reducing agent/air mixture at other times to be conveyed through at least parts of the line system of the reducing agent supply system and/or the nozzle in the cleaning operation. This has proved very particularly effective for cleaning with regard to the removal of deposits. In such case, it is advantageous if, in a further embodiment of the method, a rising and/or a falling edge of a reducing agent stream pulse falls timewise in an air stream pulse.

In a further advantageous embodiment of the method, an air stream pulse falls timewise completely in a reducing agent stream pulse. Thus, after a conveying pause with operation free from throughflow, initially the reducing agent stream, preferably suddenly, is switched on, for example by opening a shutoff valve. Once a short amount of time has elapsed, the air stream is switched on and thereafter switched off again while the reducing agent stream continues to exist. After a further in particular short amount of time, preferably in the range from one to a few seconds, the reducing agent stream is also switched off again. This produces short flushing phases with thorough flushing of the line system with liquid reducing agent, which phases precede and succeed a throughflow with reducing agent/air aerosol mixture. This has likewise proved very effective with regard to detachment of deposits.

In a further embodiment of the method, a second operating mode is provided for the cleaning operation in which a number of successive reducing agent stream pulses is produced when the air stream is switched off and a number of successive air stream pulses is produced when the reducing agent stream is switched off. This procedure has proved particularly effective with regard to the removal of particularly stubbornly adhering deposits. These are observed mainly after a relatively long stoppage of the reducing agent supply system, for example as the result of the vehicle being stationary for several hours or days. Provision may therefore be made for activation of the second operating mode generally after a pre-settable stoppage time with restarting of the reducing agent supply system.

In particular with regard to the removal of particularly stubbornly adhering fouling, it is advantageous if in a further embodiment of the invention a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle is determined and the first operating mode of the cleaning operation is activated upon exceeding a pre-settable first upper pressure threshold value and the second operating mode of the cleaning operation is activated upon exceeding a pre-settable second upper pressure threshold value, the second upper pressure threshold value being higher than the first upper pressure threshold value. In such case, it is particularly advantageous if in a further embodiment of the invention a cleaning operation of the first operating mode succeeds a cleaning operation of the second operating mode. Preferably the first operating mode is activated immediately once the second operating mode has been deactivated.

Advantageous embodiments of the invention are illustrated in the drawings and will be described below. Therein, the above-mentioned features which are still to be discussed below can be used not only in the combination of features indicated in each case, but also in other combinations or on their own, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows a diagrammatic representation of an advantageous embodiment of a reducing agent supply system,

FIG. 2 shows a schematized time diagram for a pulse sequence of air stream pulses and reducing agent stream pulses in order to make clear an advantageous variant of the method according to the invention corresponding to a first cleaning operating mode, and

FIG. 3 shows a schematized time diagram for a pulse sequence of air stream pulses and reducing agent stream pulses in order to make clear an advantageous variant of the method according to the invention corresponding to a second cleaning operating mode.

DETAILED DESCRIPTION

In the advantageous embodiment diagrammatically illustrated in FIG. 1, a reducing agent supply system 1 comprises a liquid supply line 2 for a liquid that is to be metered. A metering valve 4, which can be operated in timed manner, and an adjustable adjusting choke 5, a first pressure sensor 6 are arranged in the liquid supply line 2. The liquid supply line 2 opens, as does an air supply line 3 for supplying compressed air, into a mixing region 11 of the reducing agent supply system 1. In the mixing region 11, thorough mixing of supplied compressed air and liquid takes place such that an aerosol-like mixture with liquid droplets are produced that are as small as possible. The mixing region 11 may be formed as a separate mixing chamber or as an integral component of the liquid supply line 2 or the air supply line 3.

Below, it will be assumed that the liquid is an aqueous urea solution (HWL), which can be sprayed in the form of the aerosol-like mixture by means of a nozzle into an exhaust line of a motor vehicle upstream from what is called an SCR catalyst for removing nitrogen oxides, this not being shown in detail. The reducing agent supply system 1 is not, however, restricted to this application, but may in principle also be used for metering any other liquid additive whatsoever.

HWL is fed into the liquid supply line 2 from a storage container preferably by means of a pump, this likewise not being shown in detail. In such case, a preliminary pressure p_(HWL) is produced and/or maintained in the liquid supply line 2 upstream from the metering valve 4 by means of the pump, which pressure is detected by the first pressure sensor 6. The metering valve 4 is preferably designed in the manner of a 2/2-way solenoid valve. Provision is made for the metering valve 4 to be able to be operated such that it is either opened or closed, for which purpose it is actuated accordingly.

The adjusting choke 5 serves for setting a beneficial operating point with regard to the liquid flow rate through the opened metering valve 4 under the prevailing pressure conditions. Preferably the adjusting choke 5, as illustrated in FIG. 1, is arranged in the liquid supply line 2 upstream from the metering valve 4 and downstream from the first pressure sensor 6. It is likewise possible to arrange it at a different point of the liquid supply line 2, in particular upstream from the mixing region 11.

When the metering valve 4 is opened, HWL flows through and mixes with the supplied compressed air in the mixing region 11, or, as a result of the supplying of compressed air, is nebulized in the mixing region 11. Provision is made, in the case of normal metering operation of the reducing agent supply system 1, for compressed air to be constantly supplied, i.e., even when the metering valve 4 is closed. In particular, provision is made, upon start-up of the reducing agent supply system 1, for admetering of HWL to begin only when a sufficient air flow rate is present. Preferably a compressed-air reservoir (not shown) or a compressor can be employed for supplying compressed air. Preferably, a pre-set air pressure is set, for example, via a pressure reducer, likewise not shown. The supply of air on the entry side of the air supply line 3 can be alternatively shut off or enabled by means of a preferably succeeding actuatable switching element 16.

The mass flow of HWL through the opened metering valve 4 is mainly dependent on the differential pressure Δp present above the metering valve 4. Even with a constant preliminary pressure p_(HWL) in the liquid supply line 2, however, fluctuations in the differential pressure Δp may result due to a fluctuating metering pressure p_(L), downstream from the metering valve 4. These are caused mainly by pressure conditions in the exhaust system that fluctuate dependent on engine operation, which conditions may affect the liquid supply line 2 back to the exit side of the metering valve 4. Provision is therefore made, by means of a second pressure sensor 7, to detect the metering pressure p_(L) downstream from the metering valve 4, preferably in the mixing region 11, and thus continuously to determine the current differential pressure Δp. Arrangement of the second pressure sensor 7 at a different point downstream from the metering valve 4 is likewise possible.

In order to prevent HWL from flowing backwards into the air supply line 3, a separating element 15 is arranged in the air supply line 3 on the entry side of the mixing region 11. Preferably the separating element 15 forms the end section of the air supply line 3 and opens out into the mixing region 11. Although the separating element 15 may for example also be designed in the manner of a non-return valve, it is preferred for the separating element 15 to be formed as a nozzle, in particular as what is called a supercritical nozzle, with which an air flow of supersonic speed can be achieved. In this manner, penetration of HWL into the air supply line 3 is largely avoided. Further, strong turbulence in the mixing region 11 is permitted. Due to the air flowing at high speed into the mixing region 11, HWL supplied into the mixing region 11 is finely mixed or nebulized, and an aerosol-like mixture is produced. Furthermore, the embodiment of the separating element 15 as a supercritical nozzle permits equalization of the air flow. In order to support a uniform air flow, further provision is made to set as constant as possible an air pressure on the entry side of the air supply line 3 or upstream from the separating element 15. An excess pressure of approximately 5 bar is advantageous. Preferably, as illustrated, a third pressure sensor 14 is connected to the air supply line 3 upstream from the separating element 15, so that the air pressure prevailing upstream from the separating element 15 can likewise be monitored.

The second pressure sensor 7 together with the first pressure sensor 6 allows a determination of the differential pressure Δp present above the series connection of adjusting choke 5 and metering valve 4, and the second pressure sensor 7 in conjunction with the third pressure sensor 14 allows for a determination of the differential pressure falling above the separating element 15. The latter may, however also be determined by taking into account a pre-set air pressure, thereby dispensing with the third pressure sensor 14. As a result of the pressure monitoring, accurate metering of the mixture or of the HWL can be achieved.

The mixture produced in the mixing region 11 is supplied to the nozzle or to the exhaust aftertreatment system via a mixture line 12. The compressed air in this case serves as a transport medium. Although normally it is not preferred, it is possible also to use a different transport medium such as for example nitrogen or exhaust instead of the compressed air.

In order to control or regulate the reducing agent supply system 1, there is provided a metering control unit (not shown), to which the pressure sensors 6, 7, 14 and the metering valve 4 are connected via interfaces 8, 9, 10, 13. The metering control unit is preferably embodied in the form of a microcomputer with input/output unit, CPU and memory unit, in order to be able to process the data and measured values obtained for controlling the reducing agent supply system 1 and to be able to determine and emit corresponding control signals. For example, the switching element 16 in the air supply line 3 can be actuated via a further interface 17 such that the air supply can be alternatively switched on and off.

As a result of deposits of urea or of corrosion components, clogging may occur in the line system or alternatively in the nozzle of the reducing agent supply system 1, which clogging disrupts proper operation. Deposits typically cause constriction of the cross section in an affected line, which becomes apparent due to an increased back pressure. This can be detected for example by evaluating one or more of the pressure values supplied by the pressure sensors 6, 7, 14. A disruption caused by clogging or deposition can, however, also be detected by monitoring the air and/or HWL flow rate, which in the event of breakdown typically drop below the values provided. Below, merely by way of example, it will be assumed that clogging is present in or downstream from the mixing region 11, which is manifested by a pressure increase, detectable by the second pressure sensor 7, upon conveying air through the air supply line 3 or the mixture line 12. If in the case of the air flow rate provided an operating pressure which has risen to a first upper pressure threshold value is detected by the second pressure sensor 7, the normal metering operation is interrupted and a cleaning operation is begun. The cleaning operation involves producing an air stream formed by a number of air stream pulses and an HWL stream formed from a number of HWL stream pulses, and supplying them to the nozzle, which is explained in greater detail below with reference to FIG. 2.

FIG. 2 shows a schematized time diagram for a pulse sequence of air stream pulses 20 and HWL stream pulses 21 corresponding to a first cleaning operating mode, which is preferably activated when an operating pressure that has risen to a first upper pressure threshold value of for example 2.5 bar is detected by the second pressure sensor 7. In such case, both the air stream and the HWL stream change periodically and at least approximately abruptly from an “off” state through an “on” state and back into the “off” state again, so that approximately square-wave pulses of pre-settable length or pre-settable period result. It is particularly preferred if the air stream pulses 20 and the HWL stream pulses 21 overlap timewise. In particular, it is preferred if a rising edge 24 of an HWL stream pulse falls timewise in an active air stream pulse 20 and a falling edge 22 of an air stream pulse 20 falls in an active HWL stream pulse 21. Correspondingly, in the present case a rising edge 23 of a particular air stream pulse 20 falls in an HWL stream pulse pause, and a falling edge 25 of an HWL stream pulse 21 in an air stream pulse pause. The pulse/pause ratio of air stream pulses 20 and HWL stream pulses 21 is preferably set to approximately equal to one. The air stream pulses 20 and HWL stream pulses 21 in this case have a duration of preferably 1 s to 10 s. Provision may be made to convey a pre-settable amount of HWL of for example 50 ml in a particular HWL stream pulse.

As a result of the pulse-like conveying of air and/or HWL, deposits are effectively detached or dissolved and flushed out of the reducing agent supply system 1 via the nozzle. Generally, a duration of the cleaning operation in the manner outlined of approximately 0.5 min to approximately 10 min is sufficient for this. Either the expiry of a pre-settable duration or the pressure dropping below a pre-settable lower pressure threshold value of for example about 1.9 bar, in the present case measured by means of the second pressure sensor 7, serves as end criterion for ending the cleaning operation. Thereafter, normal metering operation can be resumed.

In particular in the case of a relatively long stoppage phase, formation of comparatively stubbornly adhering or thicker deposits may occur. This is typically manifested by a comparatively sharp rise in a back pressure, which can be measured in the reducing agent supply system 1. If, for example, an operating pressure rises to a second pre-settable pressure threshold value of about 3.5 bar as measured by the second pressure sensor 7, it is concluded that such greater degree of fouling has occurred. In this case, provision is made according to the invention to activate a cleaning operation corresponding to a second cleaning operating mode. The procedure preferably provided in this case will be explained below with reference to FIG. 3.

FIG. 3 shows, in a representation analogous to FIG. 2, a pulse sequence of air stream pulses 20 and HWL stream pulses 21 corresponding to the second cleaning operating mode. Therein, a number of HWL stream pulses 21 are carried out one after the other alternating with a number of air stream pulses 20 that are carried out one after the other, such that in the case of pulses of one type the supply of the respective other medium remains switched off. Preferably the starting point is the production of HWL stream pulses 21. This permits softening of encrustations, which can be detached in improved manner by the air stream pulses 20 succeeding the sequence of HWL stream pulses 21. A particular set of pulses may consist of from one to about 10 pulses, the pulse/pause ratio and pulse duration preferably being set analogously to the first cleaning operating mode. The cleaning operation is preferably maintained for a predetermined overall duration. However, provision may also be made to end the cleaning operation if the pressure drops below a pre-settable lower pressure threshold value, measured with active supply of air. Likewise, provision may be made to maintain the cleaning operation until a pre-settable amount of HWL has been conveyed in pulse form. This avoids an undesirably long cleaning operation.

Preferably, provision is made, directly or shortly after carrying out a cleaning operation corresponding to the second cleaning operating mode, to carry out a cleaning operation corresponding to the first cleaning operating mode illustrated in FIG. 2.

Since deposits for the most part occur to an increased extent after pauses in operation, provision may be made to begin a start-up of the reducing agent supply system 1 in principle with a cleaning operation corresponding to the first or the second cleaning operating mode. Alternatively or additionally, provision may be made to carry out a cleaning operation generally as a preventive measure once a normal metering operation has ended.

This effectively prevents deposits and/or eliminates deposits before they become evident with undesirable effects. Upon shutting down, furthermore a flushing phase may be provided in order to remove HWL residues from the line system, in which phase for a pre-settable amount of time merely an air stream is maintained. Provision may also be made generally to carry out only one cleaning operation exclusively corresponding to the first cleaning operating mode discussed above or the second cleaning operating mode.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof. 

1-10. (canceled)
 11. A method for operating a reducing agent supply system for supplying an exhaust aftertreatment system of a motor vehicle with a reducing agent for emission control, in which an air stream and a reducing agent stream are supplied to a nozzle opening into an exhaust line, the method comprising: in normal metering operation of the reducing agent supply system, producing an approximately continuous air stream and a pulsed reducing agent stream at least partially releasing the produced approximately continuous air stream and the pulsed reducing agent stream via the nozzle into the exhaust line; and in a cleaning operation for cleaning the reducing agent supply system, producing an air stream formed from a number of air stream pulses and a reducing agent stream formed from a number of reducing agent stream pulses and supplying the produced air stream and reducing agent stream to the nozzle.
 12. The method as claimed in claim 11, further comprising: determining a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle; and activating the cleaning operation upon exceeding a pre-settable, first upper pressure threshold value.
 13. The method as claimed in claim 11, further comprising: deactivating an activated cleaning operation when a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle drops below a pre-settable lower pressure threshold value, after exceeding a pre-settable total value of amount of reducing agent of the reducing agent stream formed from reducing agent stream pulses, or after exceeding a pre-settable duration from a beginning of the cleaning operation.
 14. The method as claimed in claim 12, further comprising: deactivating an activated cleaning operation when a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle drops below a pre-settable lower pressure threshold value, after exceeding a pre-settable total value of amount of reducing agent of the reducing agent stream formed from reducing agent stream pulses, or after exceeding a pre-settable duration from the beginning of the cleaning operation.
 15. The method as claimed in claim 11, wherein the cleaning operation includes a first operating mode in which a sequence of reducing agent stream pulses and air stream pulses is produced such that a particular reducing agent stream pulse overlaps in time with a particular air stream pulse.
 16. The method as claimed in claim 12, wherein the cleaning operation includes a first operating mode in which a sequence of reducing agent stream pulses and air stream pulses is produced such that a particular reducing agent stream pulse overlaps in time with a particular air stream pulse.
 17. The method as claimed in claim 13, wherein the cleaning operation includes a first operating mode in which a sequence of reducing agent stream pulses and air stream pulses is produced such that a particular reducing agent stream pulse overlaps in time with a particular air stream pulse.
 18. The method as claimed in claim 15, wherein one of a rising or a falling edge of a reducing agent stream pulse occurs during an air stream pulse.
 19. The method as claimed in claim 16, wherein one of a rising or a falling edge of a reducing agent stream pulse occurs during an air stream pulse.
 20. The method as claimed in claim 17, wherein one of a rising or a falling edge of a reducing agent stream pulse occurs during an air stream pulse.
 21. The method as claimed in claim 15, wherein an air stream pulse occurs completely during a reducing agent stream pulse.
 22. The method as claimed in claim 16, wherein an air stream pulse occurs completely during a reducing agent stream pulse.
 23. The method as claimed in claim 17, wherein an air stream pulse occurs completely during a reducing agent stream pulse.
 24. The method as claimed in claim 11, wherein the cleaning operation includes a second operating mode in which a number of successive reducing agent stream pulses is produced when the air stream is switched off and a number of successive air stream pulses is produced when the reducing agent stream is switched off.
 25. The method as claimed in claim 12, wherein the cleaning operation includes a second operating mode in which a number of successive reducing agent stream pulses is produced when the air stream is switched off and a number of successive air stream pulses is produced when the reducing agent stream is switched off.
 26. The method as claimed in claim 13, wherein the cleaning operation includes a second operating mode in which a number of successive reducing agent stream pulses is produced when the air stream is switched off and a number of successive air stream pulses is produced when the reducing agent stream is switched off.
 27. The method as claimed in claim 15, further comprising: determining a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle; activating the first operating mode of the cleaning operation upon exceeding a pre-settable first upper pressure threshold value; activating a second operating mode of the cleaning operation upon exceeding a pre-settable second upper pressure threshold value, the second upper pressure threshold value being higher than the first upper pressure threshold value, wherein when the air stream is switched off in the second operating mode of the cleaning operation a number of successive reducing agent stream pulses is produced and when the reducing agent stream is switched off in the second operating mode of the cleaning operation a number of successive air stream pulses is produced.
 28. The method as claimed in claim 16, further comprising: determining a back pressure in a line of the reducing agent supply system for transporting the air stream to the nozzle; activating the first operating mode of the cleaning operation upon exceeding a pre-settable first upper pressure threshold value; activating a second operating mode of the cleaning operation upon exceeding a pre-settable second upper pressure threshold value, the second upper pressure threshold value being higher than the first upper pressure threshold value, wherein when the air stream is switched off in the second operating mode of the cleaning operation a number of successive reducing agent stream pulses is produced and when the reducing agent stream is switched off in the second operating mode of the cleaning operation a number of successive air stream pulses is produced.
 29. The method as claimed in claim 27, wherein a cleaning operation of the first operating mode succeeds a cleaning operation of the second operating mode.
 30. The method as claimed in claim 28, wherein a cleaning operation of the first operating mode succeeds a cleaning operation of the second operating mode. 